1. Field of the Invention
The present invention relates to an encoder made of rubber material on which the S-poles and the N-poles are alternately magnetized in circumference, and a method of manufacturing the said encoder. More particularly, the present invention relates to an encoder as the before described, and, its magnetic force is strong when it is magnetized and variation and/or irregularity of magnetism in the circumferential direction of it is slight. Also the present invention relates to a method of manufacturing the said encoder.
2. Description of the Related Art
Conventionally, an encoder made of rubber material and having a magnetism has been manufactured as follows: first, an appropriate amount of magnetic powders such as ferrites are mixed into rubber material, and unvulcanized rubber compound in a sheet shape is formed by roll sheeting process using the said mixture. Next, this unvulcanized rubber compound in a sheet shape is cut into square rubber thread like a slit. This square rubber band is formed into a rubber band ring by temporarily joining the opposite ends of it together. Then, this rubber band ring is compression-molded and vulcanizingly molded from the upward and downward directions using a mold. And the S-poles and the N-poles are alternately magnetized in circumference onto this vulcanizingly molded rubber ring.
An encoder made of rubber material and having a magnetism manufactured by the above-described conventional manufacturing method has problems such as those described as follows:
When the rubber band ring is formed by temporarily joining the opposite ends of square rubber band, which was cut like a slit from sheet shaped rubber compound, the said rubber band ring is made from unvulcanized rubber compound made by mixing magnetic powders and rubber material with a little considering about the arrangement and alignment of the magnetic powders. Therefore, the vulcanizingly molded rubber ring, which was obtained by compression-molding and vulcanizingly molding the said rubber band ring causes the malfunction that the magnetic force vanes in the circumferential direction when the S-poles and the N-poles are alternately magnetized in a circumference. As a result, it was difficult to obtain an encoder made of rubber material and having a magnetism, which can produce uniform and powerful magnetic forces anywhere in the circumferential direction when it is magnetized.
Avoiding such a default, another formative method has been employed. In this formative method, unvulcanized rubber compound containing magnetic powders is formed into a thin sheet shape using a roller or an extruder for the purpose of enhancing the degree of orientation of magnetism, and a rubber band ring is formed by punching this sheet in a ring shape.
According to this method, it is possible that the arrangement of magnetic powders such as ferrites or the like is made to hold the directional orientation. Moreover, there are no joining portions, which is made by joining the opposite ends of square rubber band and making the direction of arrangement of the magnetic powders to be irregular. Therefore, one has found satisfaction to some extent.
However, in this method, it is necessary to make the sheet shaped rubber compound comparatively thin in order to enhance the degree of orientation of the magnetic powders in the process in which the sheet shaped rubber compound is formed by rolling or extruding using a roller or an extruder. Therefore, in any way, the extrusion of a sheet is troublesome. Moreover, also in the molding, many malfunctions such as the case where a plurality of sheets being superimposed has to be inputted into the roller or the extruder and the like have been occurred.
Furthermore, during the extrusion of a thin sheet, it easily engages a wrinkle, a bubble or the like. So that, when it is compression-molded, there has been a problem that the orientation of magnetism is easily dislocated.
The present invention has been made in consideration of the before described circumstances. And an object of the present invention is to provide an encoder made of rubber material and having a magnetism which can be manufactured in a simplified manufacturing step and, when the S-poles and the N-poles are alternately magnetized in circumference, whose magnetic force is strong and which exerts approximately uniform magnetism at any position in the circumferential direction, and whose variation of the magnetism is slight in the circumferential direction. Another object of the present invention is to provide a manufacturing method for manufacturing the before described encoder.
The present invention has solved the above-described problems by using a ring shaped rubber compound comprised of unvulcanized rubber into which magnetic powders are mixed and having a thickness of 1.5 times to 5 times thicker than the thickness of an encoder, which is finally produced from the said ring shaped rubber, and the said ring shaped rubber compound is hot-pressed, and vulcanizingly molded as follows. The said ring shaped rubber compound is heat compressed by using upper mold and lower mold, which pressing the said ring shaped rubber compound in the axial direction, thereby its thickness is reduced to one fifth (⅕) to two thirds (⅔), and it is flown uniformly in a large amount in the radial direction of the said ring shaped rubber compound under a high temperature.
The manufacturing steps of the present invention will be separated into each step, and described in detail with reference to the drawings as follows.
The present invention provides an encoder made of rubber material and having a magnetism for forming a pulse train and generating a signal by the magnetic force showing an example in FIG. 6, also the present invention provides its manufacturing method.
In a method that the present invention provides, first, unvulcanized rubber into which the magnetic powders are mixed is formed into a sheet shaped rubber compound having a predetermined thickness using a roller or an extruder. Alternatively, the said sheet shaped rubber compound having a predetermined thickness can be formed by extruding the said unvulcanized rubber by extruder, and then rolling an extrusion-molded unvulcanized rubber by roller.
Now, the predetermined thickness as the before described means that the thickness t1 (as shown in FIG. 1) of the sheet shaped rubber compound is 1.5 times to 5 times thicker than a thickness t (as shown in FIG. 4) of the encoder made of rubber material and having a magnetism which is the final product.
It should be noted that it is preferable that, in this step, a sheet shaped rubber compound is formed while the directional orientation of the magnetic powders is arranged. According to the present invention, the final product of the encoder made of rubber material is manufactured, succeeding to the before described step of forming a sheet shaped rubber compound having a predetermined thickness, by cutting out a ring shaped rubber compound from the said sheet shaped rubber compound, hot-pressing and vulcanizingly molding the said ring shaped rubber compound, and magnetizing the S-poles and the N-poles alternately in circumference onto the said vulcanizingly molded ring, which are explained in detail in the following.
In the before described step of forming a sheet shaped rubber compound having a predetermined thickness, if the directional orientation of the mixed magnetic powders in the unvulcanized rubber is arranged, such as regulated, a variation, such as irregularity, of magnetism in the circumferential direction of the final product of encoder, which is manufactured by and through the succeeding steps as the before described, becomes smaller. So that, in the before described step of forming a sheet shaped rubber compound, it is preferable to form a sheet shaped rubber compound with arranging and regulating the directional orientation of magnetic powders in the unvulcanized rubber.
Next, from a sheet shaped rubber compound having the afore-mentioned thickness t1, a ring shaped rubber compound 1 as shown in FIG. 1 is cut out.
Subsequently, as shown in FIG. 2, the ring shaped rubber compound 1 is inputted into molds 2a and 2b, and as shown in FIG. 3, it is heated and compressed, and vulcanizingly molded in parallel by the molds 2a and 2b along with the direction of the central axis O, that is, in the directions shown by arrows X and Y.
The heating and compression at this step is performed in the temperature range of 150xc2x0 C.-230xc2x0 C. and in the pressure range of 1 MPa-200 Mpa. And the thickness of the ring shaped rubber compound 1 is reduced to one fifth (⅕) to two thirds (⅔) of the thickness t1 of the ring shaped rubber compound 1, that is the thickness t1 is reduced to the thickness t of the encoder made of rubber material 3 which is the final product.
Owing to this, the ring shaped rubber compound 1 is pressed under a high temperature, and flown uniformly in a large amount in the radial direction of the ring shaped rubber compound 1 as shown by arrows Z and W in FIG. 1, and then formed.
Next, onto this vulcanizingly molded one, shown by the reference numeral 3 in FIG. 3, although it is not shown, the S-poles and the N-poles are alternately magnetized in circumference, and the encoder 3 as shown in FIG. 4 which is made of rubber material and having a magnetism, which is the final product, is obtained.
Thus, in a manufacturing method of the present invention, the ring shaped rubber compound 1 having the predetermined thickness (thickness corresponding to 1.5 times to 5 times of the thickness t of the encoder 3 made of rubber material which is the final product) is subjected to be compressed under a high temperature and pressed into a thickness of ⅕-⅔ of the thickness of ring shaped rubber compound. In this hot-pressing step, the ring shaped rubber compound is flown uniformly in a large amount in the radial direction shown by the arrows Z and W in FIG. 1, and then formed. Owing to this, even if there is uneven blending of the magnetic powders in the circumferential direction in the state of the ring shaped rubber compound 1, it can be removed in the step that the thickness of ring shaped rubber compound is reduced to ⅕-⅔ by the before described compression step under a high temperature. Moreover, even if there are an engaged bubble and/or wrinkle in the ring shaped rubber compound, or any variation and/or irregularity of the thickness or the like in the ring shaped rubber compound, they are also removed by this compression step under a high temperature.
Therefore, an encoder made of rubber material and having an excellent performance which can exert powerful magnetic forces, and approximately uniform magnetic forces at any position in the circumferential direction, is manufactured by magnetizing the N-poles and the S-poles alternately in circumference onto the vulcanizingly molded ring, which has been obtained by hot-pressing the ring shaped rubber compound and reducing its thickness to one fifth (⅕) to two thirds (⅔) as the before described.
An encoder made of rubber material and having a magnetism provided by the present invention is manufactured by the manufacturing method described above.
As described above, it is preferable to set the thickness t1 of the ring shaped rubber compound, which is identical with the thickness of the sheet shaped rubber compound (unvulcanized rubber) at the range between 1.5 times to 5 times thicker than the thickness t of the encoder made of rubber material, which is the final product, and to heat compress and vulcanizingly mold the said ring shaped rubber compound reducing its thickness to one fifth (⅕) to two thirds (⅔), in order to obtain and manufacture the encoder made of rubber material and having an excellent performance such as exerting powerful magnetic forces, and approximately uniform magnetic forces at any position in the circumferential direction when magnetizing the N-poles and S-poles alternately in circumference onto the vulcanizingly molded one.
It should be noted that if necessary, when the ring shaped rubber compound 1 is molded with heating and compression, a reinforcing ring 4 consisted of metals or the like as shown in FIG. 2, FIG. 3, FIG. 4, and FIG. 6 can be also inputted within the mold 2, and vulcanized and adhered at the same time.
As described above, according to the present invention, it can provide an encoder made of rubber material and having a magnetism, when the S-poles and N-poles are alternately magnetized in the circumferential direction, whose magnetic force is strong and which exerts approximately uniform magnetic force at any position in the circumferential direction and whose variation or irregularity of the magnetism is slight in the circumferential direction.